Fired bullets and bullet cartridges are left with markings from the firearm from which they come. These markings result from forced contact with metal parts in the firearm and are unique to the firearm since the metal parts have engraved surfaces which are themselves unique due to the machining process during manufacturing. Using microscopic examination of fired bullets and cartridges (casings or shells), firearms experts have traditionally been able to compare markings to identify whether fired bullets or spent cartridges originate from a given firearm.
The process of bullet examination has been successfully automated using apparatus as set out in PCT/CA92/00216 published as WO 92/20988). Bullet illumination is described in U.S. Pat. No. 5,379,355.
In manual comparison of spent cartridge casings, the cartridge casing under investigation is placed under a microscope along with a test cartridge casing from a known firearm. The markings on or in the impressions made on a cartridge casing during firing and ejection are analyzed under a microscope and compared with markings from another cartridge casing to determine if there is a match between the two cartridge casings. In the case of a cartridge casing having a primer surface, the impression of the breech face on the primer surface and the firing pin impression in the primer surface have markings that are compared. Conventionally, side illumination is used which provides an image dependent on the direction of the illumination. To compare the markings on cartridge bases, the angle of illumination must be the same. For this purpose, illumination direction protocols are used. When two cartridge casings originate from the same firearm, the markings will have the same characteristics. Since the appearance of the primer breech face impression and firing pin impression is different using different angles of illumination, the firearms examiner will usually confirm a match using illumination from more than one angle.
Images are typically taken by ballistics experts in a forensic laboratory. Computer analysis of the images to determine whether the spent cartridge casings can be traced to a known weapon is also done in the laboratory, possibly at a much later time than the image acquisition. The time elapsed from the moment when a cartridge casing is found at a crime scene to when the cartridge casing is taken to the lab may be a few hours to a few days, the greater time lapse arising when the nearest forensic lab is not located in the same city or even in a nearby city. There is also the possibility that a spent cartridge casing could be lost or misidentified as a result of shipping and transferring.